The reason is that their new, unnamed banks are having difficulty
processing the volume of orders Mt. Gox now takes in for the
digital currency. This was further complicated by Bitcoin's Wells
Fargo
getting frozen last month:

The pressure we brought onto the bank’s resources finally
gave in, and we are now working with them to find an alternative
method (hence the suspension). We would have preferred to give
notice if we were able to, but it was sudden for us as
well.

They also ask for sympathy from Bitcoin users:

Mt. Gox is certainly not a martyr, but it would be
hard to argue that we aren’t "taking one for the team" as far as
Bitcoin is concerned. We are a big target, and are
absorbing the frustrations of Bitcoiners, regulators, banks, and
a media that still doesn't quite understand what Bitcoin is. This
is a job we are happy to fill, and not just because we are
compensated for it. Our hope is that, once Bitcoin finds its
place, we will be able to say that we made a difference in
sorting it all out in the early days. New exchanges, business
models, merchants, traders, and consumers are rising up to
innovate and create a whole new way of doing business. A lot of
lawyers are getting new cars in the meantime too.